Atomic Structure Flashcards
who created the periodic table?
Dmitri Mendeleev
what was Niels Bohr’s model of the structure of an atom?
- negative electrons orbit the central nucleus in discrete orbital shells
- electrons have quantised positions and specific energies
- stable configurations have full outer shells
if atoms have similar numbers of outer shell electrons what does this mean?
they have similar properties
what is the atomic number equal to?
no. of protons and no. of electrons
what is the definition of the atomic mass unit (amu)?
equal to 1/12 of the mass of a carbon-12 atom
in imprecise terms, one amu is the average of the proton rest mass and the neutron rest mass
what is the relative atomic mass equal to?
no. of protons and neutrons
what is the charge of an element?
0 - it has equal numbers of protons and electrons
how are electron orbitals filled?
the lowest energy state is filled first
what is responsible for binding the protons and neutrons together in the nucleus?
the strong force
what happens with other isotopes of carbon, for example carbon-14?
because they are more unstable (due to the added neutrons) they are more likely to decay (beta decay)
what is carbon dating?
we can calculate how much carbon-14 was in the atmosphere a specific time relative to now which can be used in dating - by knowing the half life and working backwards
what is the order of electron orbital filling?
1s2, 2s2, 2p6, 3s2, 3p6, 4s2, 3d10, 4p6, 5s2, 4d10, 5p6, 4f14, 5d10, 6s2, 6p6
how do you calculate the average atomic weight of an element?
times each percentage abundance with the mass and add them all together
what was Thomson’s plum pudding model?
in Thomson’s model, the atom is composed of electrons surrounded by a soup of positive charge to balance the electrons’ negative charges, like negatively charged “plums” surrounded by positively charged “pudding”
what model do we use today for the structure of an atom? describe it?
quantum mechanical model - the orbitals are where we can expect to find electrons (locations are described as probabilities) and the properties of the electrons are described using a wave equation with wave and particle like properties (like a standing wave)
what is the name for numbering the electron shells around the nucleus?
principal quantum numbers (goes up to 7)
how many electrons fit in the s orbital?
2
how many electrons fit in the p orbital?
6
how many electrons fit in the d orbital?
10
how many electrons fit in the f orbital?
14
what is the magnetic quantum number (mI)?
relates to the electron’s orientation in space - it can be …-2, -1, 0, 1, 2… and so on
what is the spin quantum number (ms)?
relates to the electron’s spin - it can be -0.5 or 0.5
what is the shape of an s orbital?
spherical
what is the shape of a p orbital?
dumbell shaped
what is special about the noble gases?
they have full outer shells, complete s and p sub-shells, they tend to be very unreactive due to their stability - it won’t form primary bonds with anything, they are inert and have low energy (group 0)
what is the Pauli exclusion principle?
this states that each electron state/orbital can hold no more than two electrons and they must have opposite spin
what are valence electrons?
valence electrons are those which occupy the outermost electron shell
what determines an atom’s physical and chemical properties?
the number of valance electrons
what is electronegativity?
the ability of an atom to attract electrons to itself
what element is the most electronegative?
flourine
are metals electron donors or acceptors?
donors
are non-metals electron donors or acceptors?
acceptors
what do the groups in the periodic table relate to?
elements with the same number of valance electrons, they have similar properties
what are the 3 types of primary bonding?
ionic
metallic
covalent
what are the 2 types of secondary bonding?
van der walls
hydrogen bonding
which type of bonding is stronger, primary or secondary?
primary
what is ionic bonding and where do you find it?
- often found in ceramics like alumina Al2O3, MgO (magnesia)
- between compounds of metal and non-metal elements e.g NaCl, AL2O3, MgO, CaF2
- requires electron transfer and a large difference in electronegativity
- electrostatic attractions between oppositely charged ions
- non directional
- relatively strong
- high melting point
- high elastic modulus
- brittle (difficult for atoms to slide/rearrange)
- electrical/thermal insulators (no free electrons)
- crystal lattice structure
which is positive cation or anion? and hence which is negative?
cation - positive ion
anion - negative ion
what is covalent bonding and where do you find it?
- occurs in polymers and some ceramics (typically non-metal compounds) for example methane CH4, H2, F2, SiC
- the electrostatic attraction between a shared pair of electrons and the nuclei of both bonded atom
- the two elements have similar electronegativities
- diamond and silicon are elemental solids with covalent bonds, have larger bond energy
- insulators because there are no free electrons
- very strong, directional bond (brittle)
- high melting point
- high elastic modulus
- less dense than ionic/metallic bonding materials (directional bonds make it harder to “pack” atoms)
what is metallic bonding and where do you find it?
- occurs in metals e.g Al, Fe, Mg or tungsten (W)
- electrostatic attraction between positively charged ions and the sea of electrons surrounding them (valence electrons are delocalised)
- high electrical and thermal conductivity
- electrons are not bound to one particular atom
- non directional, intermediate strength bond
- ductile because the planes of atoms can slide over each other
- intermediate melting point and elastic modulus
- close packing of atoms (high density)
what are the two types of van der walls bonding? what is van der walls bonding?
- fluctuating/temporary dipole interactions
between inert gases or covalent molecules e.g H2 - permanent dipole-dipole interactions
between molecules with permanent dipoles like HCl or polymers
what are some characteristics of van der walls forces?
- weak, smallest bond energy out of all the bond types
- directional
- van der walls forces hold neighbouring molecules together
- seen in polymers between chains
- low stiffness
- low melting point
- very ductile
what is hydrogen bonding?
- seen in water H2O
- weak, secondary bond
- occurs from the interaction and delocalisation of hydrogen electrons
what does Fa refer to?
attractive force between two atoms - depends on the type of bonding/valence electrons (valence electron interacts with neighbouring positive nucleus)
what does Fr refer to?
repulsive force between two atoms - depends on the overlap of electrons
what is the equation for the net force between the atoms?
Fn = Fa + Fr
when do you have the equilibrium position between two atoms?
when Fa + Fr = 0 (equal and opposite) which gives the length of our bond (r = r0) - here the energy is at a minimum
is the attractive force positive or negative?
positive
is the repulsive force positive or negative?
negative
what are the forces between atoms a result of?
- the forces result from the potential energy between the atoms
- the FORCE is the DIFFERENTIAL of energy
- this energy reveals fundamental properties of materials
what is the relationship between Tm and Eo (melting point and bonding energy)?
Tm is larger if Eo is larger
what is the relationship between E and Eo (stiffness/elastic modulus and bonding energy)?
E is larger if Eo is larger
what is the general equation for the attractive force Fa?
- A / r^2 (where A is a constant for a given bond)
what is the general equation for the repulsive force Fr?
B / r^9 (where B is a constant for a given bond)
generally, what bonding do ceramics have and what is the scale of Tm and E values?
- usually ionic or covalent bonds
- large bond energy
- large Tm
- large stiffness E
generally, what bonding do metals have and what is the scale of Tm and E values?
- metallic bonding
- moderate Tm
- moderate stiffness E
generally, what bonding do polymers have and what is the scale of Tm and E values?
- covalent and secondary
- because the secondary bonding dominates they have low Tm and low stiffness E
how do we calculate equilibrium separation (r0) in nm? and how from this do we calculate bonding energy (E0) in eV?
En = Ea + Er
En = - 1.5 / r + (7x10^-6) / r^8
differentiate to get force equation
Fn = 1.5 / r^2 + (-8x7x10^-6) / r^9
Make Fn = 0 because there is 0 net force at equilibrium to find r0 in nm
substitute r0 into energy equation to get E0, bonding energy in eV
what does the bond strength and energy determine?
fundamental properties such as melting point and stiffness
what is the strength of a material dependent on?
dependent on defects within the material like chalk for example
what is the structure of diamond?
diamond is organised in a giant covalent lattice structure with strong covalent bonds between carbon atoms, each carbon atom forms 4 bonds - tetrahedral lattice
is silicon carbide SiC covalent or ionic?
covalent
what is bronze an alloy of?
Cu and Sn - copper and tin
what equations do you need to know for working out number of atoms/particles?
n = M / mr (mass over Mr) Np = n x A (Avogadro's constant)
what is Avogadro’s constant equal to?
6.022x10^23 atoms
what is the equation for % ionic character?
% ionic character = [1 - e^(-(0.250)(Xa - Xb)^2)] x 100
where:
Xa = electronegativity of most electronegative element
Xb = electronegativity of least electronegative element
